The present invention relates to a thin-film piezoelectric resonator, a filter and a duplexer including such piezoelectric resonators, and a method for producing the thin-film piezoelectric resonator.
In mobile communication apparatuses such as cellular phones which have come into wide use recently, both reduction in size and heightening in operating frequency have advanced year by year. Therefore, both reduction in size and heightening in operating frequency have been required in electronic parts used in such mobile communication apparatuses.
Some mobile communication apparatuses have a duplexer for performing switching between a transmit signal path and a receive signal path in order to use one antenna for both transmission and reception. The duplexer has a transmit filter capable of passing a transmit signal but cutting off a receive signal, and a receive filter capable of passing a receive signal but cutting off a transmit signal.
Surface acoustic wave filters have been used as the filters in the duplexer recently. Such a surface acoustic wave filter has features that the filter can support frequencies up to 2 GHz, and that the size of the filter can be reduced as compared with a ceramic filter. In the present circumstances, however, there still remain a lot of technical problems in order to apply the surface acoustic wave filter to frequencies of not lower than 2 GHz at which the mobile communication apparatuses will work in the future.
Therefore, a device called thin film bulk acoustic resonator (hereinafter referred to as FBAR) has attracted public attention recently, for example, as described in JP-A-2000-278078. The FBAR is a thin-film piezoelectric resonator using resonance in a direction of the thickness of a piezoelectric thin film. In the FBAR, the resonant frequency can be changed according to change in thickness of the piezoelectric thin film. It is conceived that the FBAR can support frequencies of the order of several GHz.
As shown in FIGS. 10A and 10B, the related-art thin-film piezoelectric resonator has a so-called coplanar structure which includes a substrate 1, upper and lower barrier layers 2 and 3 formed on upper and lower surfaces of the substrate 1 respectively, a lower electrode 4 formed on the upper barrier layer 2, a piezoelectric thin film 5 formed on the lower electrode 4, and an upper electrode 6 and ground electrodes 7 formed on the piezoelectric thin film 5. Incidentally, the substrate 1 includes a vibration space S.
K. Nakamura and H. Kobayashi “Thin Film Resonators and Filters”, International Symposium on Acoustic Wave Devices for Future Mobile Communication Systems, Mar. 5, 2001, pp. 93-99 proposes another type of the thin-film piezoelectric resonator. In that, an acoustic multi-layer film is formed without provision of any vibration space in the substrate. That is, this type of the thin-film piezoelectric resonator includes a substrate, an acoustic multi-layer film having a plurality of layers disposed on the substrate, a lower electrode disposed on the acoustic multi-layer film, a piezoelectric thin film disposed on the lower electrode, and an upper electrode disposed on the piezoelectric thin film. For example, the acoustic multi-layer film is formed by laminating layers of high acoustic impedance material such as aluminum nitride and layers of low acoustic impedance material such as silicon oxide.
In a further type of the thin-film piezoelectric resonator, a cavity is provided between the substrate and the lower electrode (JP-A-60-189307). That is, this type of the thin-film piezoelectric resonator includes a substrate, a lower electrode formed on the substrate so as to form a cavity between the substrate and the lower electrode, a piezoelectric thin film disposed on the lower electrode, and an upper electrode disposed on the piezoelectric thin film.
In each of these types of the thin-film piezoelectric resonators, a metal such as Al, Pt, Au, Ag, Cr, Cu, Ti, etc. can be used as an electrode material for forming the upper and lower electrodes which are provided on opposite surfaces of the piezoelectric thin film respectively.
Each electrode has a portion concerning a resonant portion, and a lead-out portion. The resonant portion is defined as a portion where the piezoelectric thin film is sandwiched between the lower electrode and the upper electrode, and the lead-out portion is defined as a portion except for the resonant portion. Generally, these portions are integrally formed by a thin-film forming method. Because the thickness of the resonant portion inclusive of the electrode portion is decided on the basis of a required frequency, the total thickness of the electrode inclusive of the lead-out portion is decided necessarily.
Further, international publication WO99/37023 discloses a film bulk acoustic wave device, comprising: a substrate; a bottom electrode formed on one surface of the substrate; a piezoelectric film formed on the bottom electrode; and a first top electrode formed on the piezoelectric film, further comprises a second top electrode having a larger mass load than the first top electrode, and formed on the first top electrode on the piezoelectric film when viewed from the center of the first top electrode, wherein the piezoelectric film has a high-band-cut-off-type dispersion characteristic. The cut-off frequency of a second top electrode portion piezoelectric film having a large mass load can be lower than the cut-off frequency of a first top electrode portion piezoelectric film, to thereby trap the energy of the acoustic wave in a region of the first top electrode portion side, so that good performance may be feasible
The thickness of each electrode is limited by the resonant frequency. For this reason, when each electrode is too thin, there is a possibility that electric loss may occur. Incidentally, the resonant characteristic of the thin-film piezoelectric resonator is obtained by using a thickness vibration mode of the piezoelectric thin film. Ripples due to a thickness shear mode may occur on a pass signal in the passband according to the electrode material and structure of the resonant portion.